1. Field of the Invention
This invention relates to the coating of metal articles and to electrical equipment, e.g. made of copper, silver, or aluminum, such as coils, compressors, armatures, transformers, motors, and the like, with an aqueous solution of water-soluble resins that exhibits superior resistance to fluorocarbon refrigerants, to chemical attack, and has excellent hot and cold bond strengths on helical coils, both before and after exposure to fluorocarbon refrigerants.
2. The Prior Art
In the manufacture of electrical equipment for air conditioners and refrigerators where a fluorocarbon refrigerant like fluorocarbon refrigerant 22 is used, coils and their motor components are coated with an hermetic-type varnish. Varnishes of this type have a blend of an epoxy resin and a phenolic resin dissolved in a mixture of glycol ethers or esters and an aromatic hydrocarbon solvent. In the baking operation the evaporation of the glycol ether or ester and the aromatic hydrocarbon solvent poses an air pollution problem. Control of these emissions of Volatile Organic Compounds (VOCs) to the atmosphere from industrial, stationary sources has been regulated by the Environmental Protection Agency (EPA) and by various state authorities. Typical allowed emissions are 3.0 pounds per gallon of applied coating less water. Solutions of an epoxy resin and a phenolic resin in glycol ethers or esters and aromatic hydrocarbon solvents typically contain 6.0 pounds VOC per gallon of coating.
One of the approaches to solve this problem would be to formulate a water-borne coating which eliminates the use of the aromatic hydrocarbon solvent, and reduce the requirement for organic co-solvent to meet Environmental Protection Agency regulations.
Although aqueous systems having reactive carboxylic acid groups attached to epoxy esters have been proposed in the art, they have been deficient in resistance to refrigerants and bond strengths have been low. The products are hydrolytically unstable in the liquid and cured states. Emulsions of epoxy resins have posed rheological problems and their storage stability is limited.
Examples of hermetic varnishes are disclosed in U.S. Pat. Nos. 4,433,080 and 4,454,197, which employ adducts of dihydric phenols and p-Aminobenzoic acid. These adducts are dispersed in water by means of volatile amines which form a quaternary ammonium compound, rendering them water soluble with the addition of an organic co-solvent. The resulting dispersions are too viscous to be applied above 32 weight percent solids, and they typically contain more than 3.0 pounds per gallon VOC. Additionally, because of the residual carboxylic acid groups which remain in the cured compositions after the quaternizing amine has been evaporated during curing, they exhibit poorer bond strengths after exposure to fluorocarbon refrigerants than before. In addition, this type of hermetic varnish requires extended curing cycles (e.g., 4 hours at 150.degree. C. or 2 hours at 163.degree. C.) to develop acceptable hot bond strengths, due to the need to evaporate the amine before curing can proceed to completion.
U.S. Pat. No. 3,367,991, which does not relate to hermetic varnishes, discloses adducts of epoxides of Bisphenol A cured with hexamethoxymethyl melamine curing agents, unlike the present invention which employs the combination of phenoplast alcohol and melamine-formaldehyde condensate.
U.S. Pat. No. 3,729,435 which does not relate to hermetic varnishes, relates to epoxy resins modified with dialkanolamines which are further modified with fatty acids and a polycarboxylic acid followed by mixing with aminoplasts or phenoplasts.
U.S. Pat. Nos. 4,374,233 and 4,374,965, which do not relate to hermetic varnishes, disclose epoxy/amine adducts crosslinked with aminoplasts.
U.S. Pat. No. 4,777,194 which does not relate to hermetic varnishes, relates to epoxy/amine adducts further modified by an amide-forming reaction in combination with a phenoplast.